(1) Field of the Invention
The present invention relates broadly to solar heaters which convert solar radiation into heat energy and transfer the absorbed heat either to a gas such as air or a liquid such as water, the former type being commonly referred to as a solar air heater and the latter being commonly referred to as a solar water heater. More particularly, the present invention relates to solar heaters of either type which employ a heat trap between the absorber and the light-transmitting front wall.
(2) Description of the Prior Art
Various proposals have already been made in the prior art for employing a heat trap between the absorber and front wall of either a flat plate solar heater or a transpiration solar air heater. Thus, Hollands discloses the use of a transparent honeycomb heat trap in a flat plate solar heater in an article entitled, "Honeycomb Devices in Flat Plate Solar Collectors, " Solar Energy, Vol. 9, pp. 159-169, Pergamon Press (1965). The transparent honeycomb heat trap, in this instance, serves to suppress the onset of natural convection currents and additionally reduces heat losses by radiation.
It has been further discovered that a transparent honeycomb heat trap significantly increases the overall efficiency of a transpiration solar air heater when interposed between the porous absorber and front wall as disclosed and claimed in our copending application Ser. No. 824,100 now U.S. Pat. No. 4,262,657, filed on even date herewith and assigned to the common assignee hereof, and which is a continuation-in-part of our earlier application Ser. No. 712,139 filed on Aug. 6, 1976, now abandoned. As further disclosed and claimed in our copending application, it has been found that the honeycomb heat trap, when placed in at least firm mechanical contact with the front wall, serves the additional function of providing an air buffer layer without the interposition of additional surfaces from which incident sunlight can be reflected away from the absorber and be lost.
Transparent cellular structures such as clear plastic or glass honeycombs, which are now contemplated for use as heat traps in the solar heaters described above, have been made by methods well known in the prior art. In one method that has been used heretofore, clear plastic or glass tubes are stacked and bonded together by a suitable adhesive or solvent. In another method, a multiplicity of elongated narrow strips of plastic film are first coated with an adhesive at spaced apart intervals and then adhered together. This is followed by expansion into a hexagonal honeycomb structure. The use of such bonded expanded honeycomb in a flat plate solar heater has been described in an article entitled, "Effect of a Mylar Honeycomb Layer on Solar Collector Performance," by Chun and Crandall presented at the 1974 Winter Annual Meeting of the ASME (Paper No. 74-WA/HT-11).
A disadvantage common to all honeycomb structures fabricated by the above described and other similar techniques is the presence of adhesive bonds between adjacent cells. These adhesive bonds give rise to certain problems when the honeycomb is used as a heat trap in a solar heater. One such problem is that the adhesive bonds act as scattering sites for incident light and thus reduce the overall transmission of solar rays through the honeycomb at all sun angles different from normal incidence. Thus, at all times different from solar noon, the fraction of incident sunlight which reaches the solar absorbing surface in the solar heater is reduced, with a consequent reduction in conversion efficiency. Another problem which can be associated with the use of any adhesive is that the adhesive will age with attendant cracking, embrittlement and discoloration. Furthermore, fabrication of clear plastic or glass honeycombs using an adhesive is complicated by the fact that adhesives require special handling and thus make the fabrication process time-consuming and expensive.